## in Some::Package
sub _init_attribs {
   %$fields = %{+pop};
}

## in Some::Package::Sub
my %fields = map { $_ => 1 } qw( field_a anotherone );

sub new {
    my $self = shift;
    $self->_init_attibs(\%fields);
    $self->SUPER::new(@_);
}